Apparatus and method for analysing a chemical composition of aerosol particles

ABSTRACT

The invention relates to an apparatus and a method for analysing a chemical composition of aerosol particles. The apparatus comprises an extractive electrospray ionisation source for extracting components, in particular organic compounds, from the aerosol particles and for ionising the components to ions, and a mass analyser, in particular a time of flight mass analyser, for analysing the ions, the mass analyser fluidly coupled to the extractive electrospray ionisation source. The method includes the steps of extracting components, in particular organic compounds, from the aerosol particles with an extractive electrospray ionisation source and ionising the components with the extractive electrospray ionisation source to ions, transferring the ions to a mass analyser, in particular a time of flight mass analyser, the mass analyser being fluidly coupled to the extractive electrospray ionisation source, and analysing the ions with the mass analyser.

TECHNICAL FIELD

The invention relates to an apparatus and a method for analysing achemical composition of aerosol particles.

BACKGROUND ART

Aerosols are the gaseous suspension of fine solid or liquid particleswhich are called aerosol particles. In such suspensions, gas and aerosolparticles interact with each other in the sense that gaseous substancescan condense on the surface of the aerosol particle while simultaneouslyliquid or solid substances can evaporate from the aerosol particlesurface into the gas phase. The equilibrium between the gas and theparticle phase is largely driven by the individual compound's saturationvapour pressure.

Aerosol particles usually have a size in a range from 10 nm to 10 μm.Aerosol particles smaller than 10 nm have a large surface to size ratioand therefore grow quickly into larger aerosol particles. Aerosolparticles larger than 10 μm on the other hand become too heavy to besuspended for a long time and will eventually fall to the ground. Forthis reason, the typical size range of ambient aerosol particles is from50 nm to 2000 nm or 2 μm, respectively.

Instruments for investigating the chemical composition of aerosolparticles are widely used in atmospheric sciences. However, they arealso used in health sciences in order to investigate the effect ofpollutants on human health. Pollutants are often emitted directly asparticles or gaseous compounds that may condensate onto aerosolparticles as emitted or after a chemical transformation by oxidation.The chemical analysis of aerosol particles is also an efficient way todetect traces of explosives because many explosives have low vapourpressure and therefore are hardly present in the gas phase atenvironmental temperatures but are rather present in the form of aerosolparticles. At the same time explosives present a unique sampling problembecause aerosol particles of explosives are sticky and can contaminateskin, clothes and objects of people that handle explosives.

Sampling aerosol particles has traditionally been done using filters orswabs. In this approach, the aerosol particles are collected on filtersor swabs and later analysed in an off-line procedure. Over the last 30years however, many instruments have been developed for analysing thechemical composition of aerosol particles on-line and in real-time. Mostof these instruments rely on sampling air directly into a massspectrometer and then separating the gas phase from the particle phasein several differentially pumped stages whereby the gas phase is dilutedby a factor of roughly 10¹⁰ by bringing the aerosol particles from airpressure (1000 mbar) into a vacuum with a pressure of approximately 10⁷mbar.

An example of such an apparatus and method is described in U.S. Pat. No.6,040,574 of Jayne, Worsnop and Kolb. In this apparatus and method, anaerosol particle beam is formed by passing gas with entrained aerosolparticles through a particle lens, like for example the one described inU.S. Pat. No. 5,270,542 of McMurry et al. Subsequently, the aerosolparticles are flash-vaporized on a hot surface and are ionised byelectron ionisation or another form of ionisation (e.g. vacuum ultraviolet ionisation) and then are analysed in a time-of-flight massanalyser.

U.S. Pat. No. 5,681,752 (Kimberley), U.S. Pat. No. 8,648,294 B2(Kimberley et al.)

Multiple versions of similar instruments use one or several lasers forvaporising the aerosol particles as well as for ionizing the vaporizedsubstances under high vacuum, as taught in U.S. Pat. No. 5,681,752 ofKimberley or in U.S. Pat. No. 8,648,294 B2 of Kimberley et al.

All these instruments and methods have the disadvantage that theyrequire a high vacuum and thus extensive and complex equipment.

An alternative approach for analysing the chemical composition ofaerosol particles is desorbing the aerosol particles at high pressure,for example in plasma. In this approach the gas phase of the originalgaseous suspension of aerosol particles must be exchanged with a cleangas in order to avoid background from gaseous contaminants. Thisapproach is taken in a technique called single particle inductivelycoupled plasma mass spectrometry (SI-ICP-MS) as taught for example in US2015/0235833 A1 of Bazargan et al. There, the aerosol particles aretransferred from the original gas phase either into a liquid or into aclean gas. The latter is done with a “gas exchange device” as describedby J. Anal. At. Spectrom., 2013, 28, 831-842; DOI: 10.1039/C3JA50044F orJ-SCIENCE LAB, Kyoto, Japan. Because of the use of an inductivelycoupled plasma, this approach however does not deliver information onorganic substances but only gives elemental information. Furthermore,this approach has the disadvantage that it requires extensive equipmentfor exchanging the gas phase of the original gaseous suspension ofaerosol particles with clean gas.

For the analysis of the organic compounds of aerosol particles atechnique called FIAGERO was developed where the aerosol particles arecollected on a surface. After collection the surface is heated, leadingto thermal desorption of the organic compounds which then are measuredin a chemical ionization mass spectrometer (CI-MS), like for exampledescribed by Lopez-Hilfiker et al, in Atmos. Meas. Tech., 7, 983-1001,2014. This technique is on-line but not exactly real-time since thecollection-desorption cycle takes approximately 30 minutes.

The known apparatus' and methods for analysing a chemical composition ofaerosol particles have the disadvantage that they either do not enablean on-line and real-time analysis of the chemical composition, orrequire an extensive equipment. Thus, they cannot be flexibly used fordifferent types of analyses of the chemical composition of aerosolparticles.

SUMMARY OF THE INVENTION

It is the object of the invention to create an apparatus and a methodpertaining to the technical field initially mentioned, that allow for aflexible use for analysing the chemical composition of aerosolparticles.

The solution of the invention is specified by the features of claim 1.According to the invention, the apparatus comprises an extractiveelectronspray ionisation source for extracting components, in particularorganic compounds, from the aerosol particles and for ionising thecomponents to ions, and a mass analyser, in particular a time-of-flightmass analyser, for analysing the ions, the mass analyser fluidly coupledto the extractive electronspray ionisation source.

The method according to the invention comprises the steps of extractingcomponents, in particular organic compounds, from the aerosol particleswith an extractive electronspray ionisation source and ionising thecomponents with the extractive electronspray ionisation source to ions,transferring the ions to a mass analyser, in particular a time-of-flightmass analyser, the mass analyser being fluidly coupled to the extractiveelectronspray ionisation source, and analysing the ions with the massanalyser.

In the present text, the term extractive electronspray ionisation sourceis used and has the same meaning as the term extractive electrosprayionisation source. In some cases in the present text, its abbreviationEESI is used, too. In the same sense, in the present text, the termelectronspray is used and has the same meaning as the term electrospray.

The solution has the advantage that with the extracting electronsprayionisation source the extraction and ionisation of the components of theaerosol particles as well as the ionisation of a sample comprising gasis enabled. Furthermore, in case the aerosol particles comprise organiccompounds, the organic compounds can be extracted and ionised withoutdamaging the organic compounds. Additionally, dissolving and ionisingthe aerosol particles at ambient pressure as well as at reduced pressureis enabled while at the same time an on-line and real-time analysis ofthe chemical composition of aerosol particles is enabled. Consequently,the apparatus and method according to the invention allow for a flexibleuse for analysing the chemical composition of aerosol particles.

Advantageously, the extractive electronspray ionisation source which isalso referred to as extractive electrospray ionisation source comprisesan electronspray, which is also referred to as electrospray, generatingcharged droplets of liquid for dissolving the aerosol particles.Preferably, the liquid of the charged droplets of liquid is a solventcombined with water. Alternatively, the liquid of the charged dropletsof liquid may be different from a solvent combined with water. In eithercase, in the method according to the invention, charged droplets ofliquid in which the aerosol particles are dissolved are produced by anelectronspray (or electrospray) of the extractive electronsprayionisation source (or extractive electrospray ionisation source).

Preferably, the extractive electron ionisation source comprises anevaporation unit for evaporating the liquid of the charged droplets ofliquid while maintaining the components of the dissolved aerosolparticles. In this case, in the method according to the invention, theliquid of the charged droplets of liquid are preferably evaporated bythe evaporation unit of the extractive electron ionisation source whilemaintaining the components of the dissolved aerosol particles such thatthe charge of the charged droplets is transferred to the maintainedcomponents of the dissolved aerosol particles and therefore ionising thecomponents to ions.

Advantageously, the evaporation unit is a heating unit. Advantageously,the heating unit is a heated vaporisation region in which the chargeddroplets of liquid can be directed for evaporating the liquid of thecharged droplets such that the components of the dissolved aerosolparticles remain. In an alternative however, the heating unit may be aheating element on which the charged droplets of liquid are directed forevaporating the liquid of the charged droplets of liquid. Independent ofits construction, the heating unit has the advantage that the chargefrom the charged droplets of liquid is transferred to the remainingcomponents, thus ionising the components to ions.

Alternatively to the heating unit, the evaporation unit may be anotherunit for evaporating the liquid of the charged droplets of liquid whilemaintaining the components of the dissolved aerosol particles. Forexample such another unit may be a laser or a vaporisation region with amicrowave emitter for evaporating the charged droplets of liquid suchthat the components of the dissolved aerosol particles remain.

Preferably, the mass analyser is configured to provide a mass spectrumof the ions when the ions are analysed. In this case, in the methodaccording to the invention, when analysing the ions with the massanalyser, preferably a mass spectrum of the ions is produced. This hasthe advantage that the analysis of the ions is simplified.Alternatively, the mass analyser may not be configured to provide a massspectrum of the ions when analysing the ions.

Advantageously, the apparatus is a single particle aerosol massspectrometer. In this case, in the method according to the invention,the aerosol particles are preferably each analysed individually byextracting and ionising with the extractive electronspray ionisationsource the components of each aerosol particle individually andsubsequently transferring for each aerosol particle individually theobtained ions to the mass analyser and analysing the obtained ions withthe mass analyser. Thus, the apparatus advantageously comprises acontrol unit for triggering the mass analyser whenever an individualaerosol particle reaches the extractive electronspray ionisation source,triggering the mass analyser for analysing the ions originating from theindividual aerosol particle. For this analysis of the ions originatingfrom one individual aerosol particle, the ions produced by theextractive electronspray ionisation source are preferable extracted intothe mass analyser in a burst of ion extractions for the analysis.

Independent of whether the apparatus is a single particle aerosol massspectrometer or not, the apparatus further preferably comprises anelectronic data acquisition system for processing signals provided bythe mass analyser. This electronic data acquisition system preferablycomprises at least one analog-to-digital converter (ADC) producingdigitized data from the signals obtained from the mass analyser and afast processing unit receiving the digitized data from saidanalog-to-digital converter, wherein the fast processing unit isprogrammed to continuously, in real time inspect the digitized data forevents of interest measured by the mass analyser, and wherein theelectronic data acquisition system is programmed to forward thedigitized data representing mass spectra relating to events of interestfor further analysis and to reject the digitized data representing massspectra not relating to events of interest.

This has the advantage that a high data acquisition speed can beachieved.

In particular, the digitized data is constituted by (or comprises) massspectra, for simplicity, in the following this term is used for spectraof values of m/Q. (mass/charge). The fast processing unit may comprisein particular a digital signal processor (DSP), most preferably a FieldProgrammable Gate Array (FPGA).

Continuous, real-time processing means that essentially all incomingdata obtained from the ADC may be readily inspected for events ofinterest prior to deciding about forwarding or rejecting the data, thetime used for inspection of a certain portion of data being equal orless than the time used for obtaining the signals represented by thedata portion by the mass analyser. In case the mass analyser is atime-of-flight mass analyser, the mass analyser may be configured tocontinuously acquire time-of-flight (TOF) extractions. In this case,simultaneous to the continuous acquisition of TOF extractions, the fastprocessing unit is preferably used for real-time analysis of the data toidentify regions within the continuous stream of TOF extractions thatcontain events of interest. This is of particular interest for a singleparticle aerosol mass spectrometer where each time when an aerosolparticle is ionised by the extractive electronspray ionisation sourcecan be detected by the fast processing unit by identifying regionswithin the continuous stream of TOF extractions that contain events ofinterest in the form of a signature of an ionised aerosol particle.

We refer to those instances when a sample of interest is present asevents or events of interest. We refer to the inventive method as “eventtriggering”.

Rejection of digitized data not relating to events of interest meansthat this data is not forwarded to the usual further analysis. It may becompletely discarded, or processed in a way that does not use asubstantial capacity of the communication channel linking the electronicdata acquisition system to the hardware performing the further analysis.A corresponding processing may include heavy data compression, inparticular lossy compression as achieved by further processing,especially on-board at the fast processing unit.

Since the maximum continuous save rate (MCSR) of existing technologiesis limited by overhead processes, the data rate for rapidly occurringevents increase to a level that is too large to handle for today's datasystems, whose bottle necks are given in particular by the downloadspeed from the DAG to the PC, the processing of the data in the PC, orthe writing of the data to the mass storage device. The MCSR, in turn,limits the maximum rate at which events can occur and still beindividually saved with high efficiency.

Event triggering circumvents these overhead bottlenecks by transferringand saving only select TOF extractions that correspond to events ofinterest (EOIs). That is, TOF data are continuously acquired but not alldata are transferred and saved.

Event triggering allows for maintaining efficiency at high speed byeliminating all processing times (idle time in acquisition) for datasegments that do not contain information about events. By reducing deadtimes, reducing PC data load, and increasing the fraction of events thatmay be recorded at high rates, the device allows for improving TOFperformance for experiments targeting both steady-state and time-varyingcharacterization of samples.

In particular, the data acquisition with event triggering enables highlyefficient data acquisition at rates faster than the MCSR for experimentsmeasuring multiple successive samples (discontinuous), i. e. cases wherethe signal of interest is oscillating between ON states (sample present)and OFF states (time between sample). It basically allows for measuringthe complete chemical composition of many events in rapid successionwith a TOFMS. Thus it is particularly advantageous in case the apparatusis single particle aerosol mass spectrometer.

Furthermore, event triggering is particularly preferable in systems formeasuring successive samples that are introduced to the massspectrometer in a rapid and non-periodic or non-predictable manner, i.e. occurrences of successive events are not strictly periodic in timeand external triggering of the TOF is not possible and/or practical. Inthese and other cases, averaging of data may be difficult and/or lackmeaning. A highly relevant example of non-periodical, inhomogeneousevents is the measurement of the chemical composition of individualsmall particles, for example nano particles, aerosol particles, cells orother biological entities, clusters and other entities with a dimensionfalling in the range of 1 nm or larger. In such cases, particles arerapidly sampled into the mass spectrometer in a sporadic succession.

Further details on the event triggering are described in WO 2016/004542A1 of Tofwerk AG.

Preferably, the apparatus comprises an ion mobility spectrometer whichis arranged between the extractive electronspray ionisation source andthe mass analyser and fluidly coupled to the extractive electronsprayionisation source and the mass analyser. This has the advantage that theions can be separated according to their mobility before transferringthem to the mass analyser. Thus, the analysis of samples of aerosolparticles comprising a large number of different substances issimplified. In particular, due to the ion mobility spectrometer, ions ofsubstances having different mobilities reach the mass analyser atdifferent times. Consequently, one mass spectrum obtained with the massanalyser comprises signals from ions of a smaller number of differentsubstances. Therefore, the identification of the different substances inthe particular mass spectrum is simplified. Yet, the different massspectra obtained during the time interval the ions of substances havingdifferent mobilities reach the mass analyser provide the totalinformation of the substances in the sample.

Alternatively, the apparatus may not comprise an ion mobilityspectrometer.

Advantageously, the apparatus comprises an inlet for passing a gaseoussuspension of the aerosol particles from an outside of the apparatusinto the apparatus for analysing the chemical composition of the aerosolparticles, the inlet being fluidly coupled to the extractiveelectronspray ionisation source. This has the advantage that a sampleconsisting of a gaseous suspension of the aerosol particles can beanalysed. Thus, an on-line analysis of the chemical composition ofaerosol particles is enabled.

In a preferred variant, the inlet is an inlet for ambient air, thus aninlet for passing ambient air comprising the aerosol particles from theoutside of the apparatus into the apparatus for analysing the chemicalcomposition of the aerosol particles. This variant has the advantagethat the chemical composition of aerosol particles dispersed in theambient air can be analysed. In this case, the aerosol particles may forexample be or comprise pollutants or be or comprise explosives. Inanother variant however, the inlet may be an inlet for passing adifferent gaseous suspension of the aerosol particles than ambient aircomprising the aerosol particles form the outside of the apparatus intothe apparatus for analysing the chemical composition of the aerosolparticles.

Alternatively, the apparatus may not comprise such an inlet. In such analternative, the aerosol particles may be passed differently to theextractive electron ionisation source.

Preferably, the apparatus comprises a denuder line which fluidly couplesthe inlet to the extractive electronspray ionisation source, the denuderline comprising a denuder for separating the aerosol particles from thegas phase of the gaseous suspension when passing from the inlet to theextractive electronspray ionisation source. This has the advantage thatonly the aerosol particles reach the extractive electronspray ionisationsource and are ionised by the extractive electronspray ionisation sourcesuch that the mass spectra obtained with the mass analyser only containsignal from ions originating from the aerosol particles. Since the gasdoes not reach the extractive electronspray ionisation source, the massspectra do not contain signal from ions originating from the gas.Consequently, mass spectra obtained with the denuder in place comprisesignal from ions of a smaller number of different substances. Therefore,the identification of the different substances in the mass spectra issimplified.

In an alternative, the apparatus may not comprise such a denuder line.

Advantageously, the apparatus comprises a filter line which fluidlycouples the outside of the apparatus with the extractive electronsprayionisation source, the filter line comprising a filter for filtering theaerosol particles out of the gaseous suspension and thus separating thegas phase from the aerosol particles when passing from the outside ofthe apparatus to the extractive electronspray ionisation source. Ifthere is a denuder switched in series with the filter, i.e. in case adenuder is switched upstream or downstream of the filter, the filterline with the filter has the advantage that a rapid determination of abackground spectrum is enabled which can be subtracted from a spectrumobtained from the aerosol particles, thus reducing the detection limit.In case there is no denuder switched in series with the filter, i.e. incase there is no denuder switched upstream or downstream of the filter,the filter line with the filter has the advantage that only the gasreaches the extractive electronspray ionisation source and is ionisedsuch that the mass spectra obtained with the mass analyser only containsignal from ions originating from the gas. Since the aerosol particlesdo not reach the extractive electronspray ionisation source, the massspectra do not contain signal from ions originating from the aerosolparticles. Consequently, mass spectra obtained with the filter in placecomprise signal from ions of a smaller number of different substances.Therefore, the identification of the different substances in the massspectra is simplified.

In one variant, the filter line fluidly connects the inlet with theextractive electronspray ionisation source. In another variant, theapparatus comprises a second inlet for passing the gaseous suspension ofthe aerosol particles into the apparatus for analysing the chemicalcomposition of the aerosol particles, wherein the filter line fluidlyconnects the second inlet with the extractive electronspray ionisationsource. In this case, in a preferred variant, the second inlet is aninlet for ambient air, thus an inlet for passing air comprising theaerosol particles from the outside of the apparatus into the apparatusfor analysing the chemical composition of the aerosol particles. Thisvariant has the advantage that the chemical composition of the gas phaseof ambient air can be analysed. In another variant however, the secondinlet may be an inlet for passing a different gaseous suspension of theaerosol particles than ambient air form the outside of the apparatusinto the apparatus for analysing the chemical composition of the aerosolparticles.

In case the apparatus comprises the denuder line and the filter line,the denuder line preferably enables the aerosol particles to bypass thefilter, while the filter line preferably enables the gas phase of thegaseous suspension to bypass the denuder. Thus, the denuder line and thefilter line are preferably switched in parallel. For example, thedenuder line and the filter line may be fluidly connected by one or moreswitchable valves. By controlling these one or more switchable valves,the gaseous suspension of the aerosol particles may be directed via thedenuder line or via the filter line from the outside of the apparatus tothe extractive electronspray ionisation source. Thus, either the aerosolparticles or the gas phase can be transferred from the outside of theapparatus to the extractive electronspray ionisation source forionisation and subsequent analysis with the mass analyser. In case thegas phase is transferred to the extractive electronspray ionisationsource for ionisation, the extractive electronspray ionisation sourcemay be called a secondary electro spray ionization source.

The apparatus comprising the denuder line and the filter line, whereinthe denuder line enables the aerosol particles to bypass the filter,while the filter line enables the gas phase of the gaseous suspension tobypass the denuder, has the advantage that the identification of thedifferent substances in the mass spectra is simplified while yet, overthe mass spectra obtained from the ions of the aerosol particles and themass spectra obtained from the ions of the gas, the total information ofthe substances in the sample can be gathered. This advantage isparticularly present in case substances in the aerosol particles andsubstances in the gas shall be analysed and in case the sample comprisesa large number of different substances because the number of substancesproviding signal in one mass spectrum can be reduced as it is possibleto provide either only aerosol particles or only gas to the extractiveelectronspray ionisation source and thus provide either only ionsoriginating from the aerosol particles or only ions originating from thegas to the mass analyser. This reduction of the number of substancesproviding signal in one mass spectrum is particular advantageous incases where a large variety of substances should be detected in thesample. This is for example the case if the apparatus is used fordetecting explosives since some components of explosives arepredominantly present in the gaseous phase while other are predominantlypresent in the form of aerosol particles or in condensed form on someother aerosol particles. Thus, when using the method according to theinvention for detecting explosives in a sample comprising explosives, itis an advantageous embodiment to employ the denuder line and the filterline, wherein the denuder line enables the aerosol particles to bypassthe filter, while the filter line enables the gas phase of the gaseoussuspension to bypass the denuder.

In the case the sample comprises a very large number of differentsubstances, the apparatus preferably further comprises an ion mobilityspectrometer which is arranged between the extractive electronsprayionisation source and the mass analyser and fluidly coupled to theextractive electronspray ionisation source and the mass analyser. Thecombination of the ion mobility spectrometer with the denuder line andthe filter line, wherein the denuder line enables the aerosol particlesto bypass the filter, while the filter line enables the gas phase of thegaseous suspension to bypass the denuder, has the advantage that theidentification of the different substances in a particular mass spectrumis further simplified while yet, over all mass spectra, the totalinformation of the substances in the sample can be gathered. Again, thisis for example the case the apparatus is used for detecting explosives.Nonetheless, an apparatus without an ion mobility spectrometer may beused for detecting explosives, too, or even an apparatus withoutparallel switched denuder line and filter line may be used for detectingexplosives.

In a variant however, the denuder line and the filter line may beswitched in series.

Alternatively, the apparatus may not comprise such a filter line.

Preferably, the apparatus comprises a clean gas line for fluidlycoupling a clean gas source via the denuder with the extractiveelectronspray ionisation source. The clean gas is preferably a pure gas.The pure gas has preferably no hydrocarbon contamination. For example,the clean gas may be Argon or Nitrogen.

The clean gas line may comprise a switchable valve for separating theclean gas source from the denuder or fluidly coupling the clean gassource to the denuder. Independent on whether the clean gas linecomprises such a switchable valve or not, the clean gas line has theadvantage that clean gas can be passed through the denuder to theextractive electronspray ionisation source and thus to the massanalyser, thus enabling to check the cleanliness and integrity of thedenuder.

Alternatively, the apparatus may not comprise such a clean gas line.

Advantageously, the apparatus comprises a test gas line for fluidlycoupling a test gas source via the denuder with the extractiveelectronspray ionisation source. The test gas is preferably purenitrogen with 10 ppm of benzene, toluene and xylene each, which issometimes called BTX. In a variant, the test gas may however be adifferent gas.

The test gas line may comprise a switchable valve for separating thetest gas source from the denuder or fluidly coupling the test gas sourceto the denuder. Independent on whether the test gas line comprises sucha switchable valve or not, the test gas line has the advantage that testgas can be passed through the denuder to the extractive electronsprayionisation source and thus to the mass analyser, thus allowing toregenerate the denuder before its performance deteriorates and theapparatus produces data with high background and therefore lowsensitivity.

Alternatively, the apparatus may not comprise such a test gas line.

Preferably, the apparatus comprises an air line for fluidly connectingan air source with the extractive electronspray ionisation source. Thisair source may be a container of air in atmospheric or higher pressureor may be air on the outside of the apparatus. In case the apparatuscomprises a denuder, this air line preferably bypasses the denuder. Incase the apparatus comprises a filter line, the air line preferablybypasses the filter. The air line may comprise a switchable valve forseparating the air source from the extractive electronspray ionisationsource or fluidly coupling the air source to the extractiveelectronspray ionisation source.

The air line has the advantage that running the apparatus in a modewhere trace gas components are measured is enabled. Furthermore, the airline has the advantage that air can be fed directly to the extractiveelectron ionisation source in order to check the integrity of the gasmeasuring mode, i.e. the mode where the gaseous suspension of theaerosol particles is directed via the filter line from the outside ofthe apparatus to the extractive electronspray ionisation source.

Alternatively, the apparatus may not comprise such an air line.

Preferably, the apparatus comprises a calibration gas line for fluidlyconnecting a calibration gas source with the extractive electronsprayionisation source. The calibration gas is preferably nitrogen containinga known amount of the gas one plans to measure. It may however as wellbe pure nitrogen with accurately known amount of benzene, toluene andxylene, which is sometimes called BTX. The amounts of benzene, tolueneand xylene may however differ from the amounts possible used as testgas.

In case the apparatus comprises a denuder, this calibration gas linepreferably bypasses the denuder. In case the apparatus comprises afilter line, the calibration gas line preferably bypasses the filter.The calibration gas line may comprise a switchable valve for separatingthe calibration gas source from the extractive electronspray ionisationsource or fluidly coupling the calibration gas source to the extractiveelectronspray ionisation source. Independent of whether the calibrationgas line comprises such a switchable valve or not, the calibration gasline has the advantage that calibration gas can be passed to theextractive electronspray ionisation source and thus to the mass analyserfor calibrating the apparatus for enabling quantitative measurementslike for example quantitative air sample measurements.

Preferably, the apparatus comprises a second ionisation source, inparticular a spark discharge ionisation source, wherein the secondionisation source is parallel connected to the extractive electronsprayionisation source. This has the advantage that with the secondionisation source, different types of ions can be generated than withthe extractive electronspray ionisation source. Consequently, anelemental analysis of the chemical composition of the aerosol particlesis enabled.

In case the apparatus comprises a second ionisation source, theapparatus advantageously comprises a switchable valve fluidly connectedto the extractive electronspray ionisation source and the secondionisation source, wherein the switchable valve is switchable between afirst position where the aerosol particles are passed to the extractiveelectronspray ionisation source for ionisation and a second positionwhere the aerosol particles are passed to the second ionisation sourcefor ionisation. Advantageously, the mass analyser is fluidly coupled tothe extractive electronspray ionisation source and to the secondionisation source. This has the advantage that ions produced by theextractive electronspray ionisation source and ions produced by thesecond ionisation source can be analysed with the mass analyser. In avariant however, the apparatus comprises a second mass analyser which isfluidly coupled to the second ionisation source. In this variant, ionsproduced by the extractive electronspray ionisation source can beanalysed by the mass analyser, while ions produced by the secondionisation source can be analysed by the second mass analyser.

The second ionisation source being a spark discharge ionisation sourcehas the advantage that the aerosol particles can be atomised andionised. Thus, the metal content of the aerosol particles can beoptimally identified.

In case the second ionisation source is a spark discharge ionisationsource and the apparatus comprises a denuder line, the apparatusadvantageously comprises a bypass line for bypassing the denuder in thedenuder line. This has the advantage that a gaseous suspension ofaerosol particles can be transmitted directly to the second ionisationsource for ionisation, thus enabling a better identification of themetal content of the aerosol particles.

In case the second ionisation source is a spark discharge ionisationsource, the second ionisation source preferably comprises an electrodefor collecting the aerosol particles and a plasma source for generatingplasma from the aerosol particles. The plasma source may for example bea microwave source for generating pulsed microwaves. In an example, thepulsed microwaves may have a frequency of 2.45 GHz and a peak power of 3kW. Further details of such a spark discharge ionisation source aredisclosed in U.S. Pat. No. 9,091,597 B2 of Kulkami et al.

Alternatively, the apparatus may not comprise a second ionisationsource.

In case the apparatus comprises a second ionisation source, theapparatus preferably comprises a second calibration gas line for fluidlyconnecting a second calibration gas source with the second ionisationsource. The second calibration gas preferably comprises metal nanoparticles.

Alternatively, the apparatus may not comprise such a second calibrationgas line.

Advantageously, the apparatus is an apparatus for determining an aerosolparticle size distribution and comprises an aerosol particle gate and adrifting region, wherein the drifting region is fluidly coupled to theaerosol particle gate and the aerosol particle gate is configured toinsert batches of aerosol particles or individual aerosol particles intothe drifting region for passing the aerosol particles through thedrifting region, wherein the mass analyser is fluidly coupled to thedrifting region and configured to measure the drift time the aerosolparticles require to pass through the drifting region. This has theadvantage that a more profound analysis of the aerosol particles isenabled. In a preferred variant, the drifting region is arranged in thedenuder line between the denuder and the extractive electronsprayionisation source.

Alternatively, the apparatus may not be an apparatus for determining anaerosol particle size distribution.

In case the apparatus is an apparatus for determining an aerosolparticle size distribution, the apparatus and the method may employ amultiplexing apparatus for determining the aerosol particle sizedistribution as described in WO 2014/026298 A1 of Tofwerk AG.

Preferably, the method according to the invention is used in a methodfor detecting explosives in a sample comprising aerosol particles. Inthis case, the method for detecting explosives in a sample comprisingaerosol particles comprises the steps of analysing the sample with themethod according to the invention, and of examining whether the resultobtained by the analysis with the method according to the inventionprovides indications of the presence of an explosive.

Other advantageous embodiments and combinations of features come outfrom the detailed description below and the totality of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings used to explain the embodiments show:

FIG. 1 a schematic view of an apparatus according to the invention foranalysing a chemical composition of aerosol particles, which is as wellused to illustrate the method according to the invention.

In the FIGURE, the same components are given the same reference symbols.

PREFERRED EMBODIMENTS

FIG. 1 shows a schematic view of an apparatus 1 according to theinvention for analysing a chemical composition of aerosol particles. Themethod according to the invention for analysing a chemical compositionof aerosol particles is explained in the context of the same FIGURE.

The apparatus 1 comprises an extractive electronspray ionisation source2 and a mass analyser 3 in the form of a time of flight (TOF) massanalyser. The apparatus 1 further comprises a second ionisation source 5in the form of a spark discharge ionisation source. This secondionisation source 5 is parallel connected to the EESI source 2. The massanalyser 3 is fluidly coupled to the EESI source 2 and to the secondionisation source 5 via an interface 4 in order to analyse ions producedby the EESI source 2 and ions produced by the second ionisation source5.

The apparatus 1 comprises a switchable valve fluidly connected to theEESI source 2 and the second ionisation source 5, wherein the switchablevalve is switchable between a first position, where the aerosolparticles are passed to the EESI source 2 for ionisation, and a secondposition, where the aerosol particles are passed to the secondionisation source 5 for ionisation. This switchable valve may be thelater described second valve 32. It may however be a different valvewhich is located downstream of the second valve 32.

The EESI source 2 comprises an electronspray generating charged dropletsof liquid for dissolving the aerosol particles, wherein the liquid ofthe charged droplets of liquid is a solvent combined with water.Furthermore, the EESI source 2 comprises an evaporation unit forevaporating the liquid of the charged droplets of liquid whilemaintaining the components of the dissolved aerosol particles. In thepresent embodiment, the evaporation unit is a heating unit comprising aheated vaporisation region in the form of a heated capillary or ionguide in which the charged droplets of liquid can be directed forevaporating the liquid of the charged droplets such that the componentsof the dissolved aerosol particles remain.

The mass analyser 3 is configured to provide a mass spectrum of the ionswhen the ions are analysed.

When employing the method according to the invention, the aerosols arepassed to the EESI source 2 for extracting components, in particularorganic compounds, from the aerosol particles with an EESI source 2 andionising the components with the EESI source 2 to ions. Furthermore, theions are transferred to the mass analyser 3 and analysed with the massanalyser 3. The method according to the invention can be used in amethod for detecting explosives in a sample comprising aerosolparticles. In this case, the method for detecting explosives in a samplecomprising aerosol particles comprises the steps of analysing the samplewith the method according to the invention, and of examining whether theresult obtained by the analysis with the method according to theinvention provides indications of the presence of an explosive.

The apparatus 1 comprises an inlet 6 for passing a gaseous suspension ofthe aerosol particles from an outside of the apparatus 1 into theapparatus for analysing the chemical composition of the aerosolparticles. In the present embodiment, the inlet 6 is an inlet forambient air, thus an inlet for passing ambient air comprising theaerosol particles from the outside of the apparatus 1 into the apparatus1 for analysing the chemical composition of the aerosol particles. Wealso call the ambient air with the aerosol particles sample gas.

Furthermore, the apparatus 1 comprises a denuder line 11 which fluidlycouples the inlet 6 to the EESI source 2, the denuder line 11 comprisinga denuder 21 for separating the aerosol particles from the gas phase ofthe gaseous suspension when passing from the inlet 6 to the EESI source2.

The ambient air is scrubbed from gaseous trace gases by passing adenuder 21, which greatly reduces the background otherwise caused bysuch gaseous contaminants. From there the aerosol particles continueinto the EESI source 2 where the organic compounds of the aerosolparticles are extracted into the liquid phase by directing anelectrospray into the air flow. Afterwards the droplets evaporate duringtransit through a heated capillary or ion guide. The solvent is therebyrapidly removed while the droplets' charge remains in the droplets. Thedroplets eventually explode into smaller droplets by Coulomb explosionmechanism and in the end charging the organic molecules of interest. Theions proceed through the interface 4 which is differentially pumped intothe mass analyser 3. The mass analyser 3 can also be an ion mobilityanalyser coupled to a mass analyser. Note that the interface 4 can alsoinclude ion optics for parent ion selection and fragmentation. An iondetector within the mass analyser will generate electric signals, whichare recorded by data acquisition electronics (not shown) which digitizethe data and send it to a computer (not shown).

The apparatus 1 further comprises a filter line 12 which fluidly couplesthe outside of the apparatus 1 with the EESI source 2, the filter line12 comprising a filter 22 for filtering the aerosol particles out of thegaseous suspension and thus separating the gas phase from the aerosolparticles when passing from the outside of the apparatus 1 to the EESIsource 2. More precisely, the filter line 12 fluidly connects the inlet6 with the EESI source 2. Thus, obtaining mass spectra of the gaswithout any aerosol particles is enabled. Therefore, the identificationof the different substances in the mass spectra is simplified becausedue to the filter line 12 with the filter 22 and the parallel switcheddenuder line 11 with the denuder 21, mass spectra from ions of eitherthe gas or the aerosol particles can be obtained while yet, over themass spectra obtained from the ions of the aerosol particles and themass spectra obtained from the ions of the gas, the total information ofthe substances in the sample can be gathered. This advantage isparticularly present in case substances in the aerosol particles andsubstances in the gas shall be analysed and in case the sample comprisesa large number of different substances because the number of substancesproviding signal in one mass spectrum can be reduced as it is possibleto provide either only aerosol particles or only gas to the EESI source2 and thus provide either only ions originating from the aerosolparticles or only ions originating from the gas to the mass analyser 3.This reduction of the number of substances providing signal in one massspectrum is particular advantageous in cases where a large variety ofsubstances should be detected in the sample. This is for example thecase if the apparatus 1 is used for detecting explosives since somecomponents of explosives are predominantly present in the gaseous phasewhile other are predominantly present in the form of aerosol particlesor in condensed form on some other aerosol particles. Thus, when usingthe method according to the invention for detecting explosives in asample comprising explosives, it is an advantageous embodiment to employthe denuder line 11 and the filter line 12, wherein the denuder line 11enables the aerosol particles to bypass the filter 22, while the filterline 12 enables the gas phase of the gaseous suspension to bypass thedenuder 21. Furthermore, the apparatus comprises a valve 31 in thedenuder line 11 upstream of the denuder 21 which can be switched toallow the sample gas to take the filter line 12 into the filter 22 whichremoves the aerosol particle. This will result in particle free airwhich is then again passed through the denuder 21 and the EESI source 2.This allows rapid determination of a background spectrum which can besubtracted from the particle spectrum, further reducing the detectionlimit of the method and the apparatus 1.

The apparatus 1 comprises a clean gas line 13 for fluidly coupling aclean gas source via the denuder 21 with the EESI source 2. The cleangas is preferably a pure gas with no hydrocarbon contamination. Forexample, the clean gas is Argon or Nitrogen. This allows for easilyassessing the background and cleanliness of the system and allowsidentifying problems fast. Furthermore, the apparatus 1 comprises a testgas line 14 for fluidly coupling a test gas source via the denuder 21with the EESI source 2. The test gas is preferably pure nitrogen with 10ppm of benzene, toluene and xylene each, which is sometimes called BTX.In a variant, the test gas may however be a different gas. Using BTX astest gas however allows routine assessment of the denuder gas phaseremoval efficiency since it contains a high concentration of organic gascontaminants. If the gas phase removal efficiency is not >99% thedenuder should preferably be regenerated.

Both the clean gas line 13 and the test gas line 14 are fluidly coupledvia valve 31 with the denuder line 11 and the denuder 21. It is to bementioned here that valve 31 symbolizes a valve or a combination ofvalves which allow only one line of the incoming lines denuder line 11,filter line 12, clean gas line 13 and test gas line 14 to be connectedto the denuder 21 at any time.

The apparatus 1 additionally comprises a second valve 32 in the denuderline 11 downstream of the denuder 21 which can be switched to allow thesample gas or aerosol particles coming from the denuder 21 to reach theEESI source 2 or to allow gas from another gas line to reach the EESIsource 2 without having passed the denuder 2. In the present embodiment,both, a branch of the clean gas line 13 and a branch of the test gasline 14 are fluidly connected with the second valve 32. Thus, clean gasand test gas can be passed to the EESI source 2 while bypassing thedenuder 21. Passing clean gas to the EESI source 2 while bypassing thedenuder 21 for example enables to check the cleanliness of the system inthe gas mode and verify the primary spray fluid in the EESI source 2 isnot contaminated.

Furthermore, the apparatus 1 comprises a bypass line 15 for bypassingthe denuder 21 in the denuder line 11. This bypass line 15 is fluidlyconnected with the denuder line 11 upstream of the denuder 21 andfluidly connected with the second valve 32. Thus, sample gas withaerosol particles can be passed to the EESI source 2 while bypassing thedenuder 21 and the filter 22. This bypass line 15 can also be called anair line for fluidly connecting an air source with the extractiveelectronspray ionisation source. Thus the apparatus 1 can be operated ina gas sampling mode and a particle sampling mode by switching the secondvalve 32.

The apparatus 1 additionally comprises a calibration gas line 16 forfluidly connecting a calibration gas source with the EESI source 2. Thiscalibration gas line 16 is fluidly connected to the second valve 32which is switchable between a mode where the calibration gas is directedto the EESI source 2 and a mode where no calibration gas is directed tothe EESI source 2. The calibration gas is preferably nitrogen containinga known amount of the gas one plans to measure. It may however as wellbe pure nitrogen with accurately known amount of benzene, toluene andxylene, which is sometimes called BTX. The amounts of benzene, tolueneand xylene may however differ from the amounts possible used as testgas. In case the calibration gas comprises known amounts of organictraces, a calibration of the measurements in the gas mode when air isallowed through the bypass line 15, and also to some degree in theparticle mode when the air sample with aerosol particles is allowedthrough the denuder line 11 and the denuder 21.

It is to be mentioned here that the second valve 32 symbolizes a valveor a combination of valves which allow only one line of the incomingclean gas line 13, calibration gas line 16, bypass line 15 and denuderline 11 with denuder 21 to be connected to the EESI source 2 at anytime.

Both the valve 31 and the second valve 32 should be designed in a waythat prevents the loss of aerosol particles during wall collisions. Thisis preferably done by maintaining a laminar gas flow which is part ofthe knowhow of experts in the field of aerosol measurement.

The second valve 32 needs to be designed in a way that prevents thecondensation of organic gases on walls. This is done by using theappropriate materials for the lines and the valve 32, and by maintaininga laminar gas flow which all is part of the knowhow of experts in thefield of aerosol measurement. In a variant, it is as well possible toheat the lines and the second valve 32 in order to prevent condensationof organic gases on walls.

The apparatus 1 comprises a second calibration gas line 17 for fluidlyconnecting a second calibration gas source with the second ionisationsource 5. The second calibration gas preferably comprises metal nanoparticles. In the embodiment shown in FIG. 1, the second calibration gasline 17 comprises a third valve 33 which enables to control the flow ofthe second calibration gas to the second ionisation source 5. Thefunction of this third valve 33 may however be integrated in the secondvalve 32. In this case, the second calibration gas line 17 would beconnected to the second valve 32.

In the present embodiment, where the second calibration gas line 17 isfluidly connected via the third valve 33 to the second ionization source5, the apparatus 1 further comprises a second bypass line 18 connectingthe inlet 6 directly with the third valve 33. By this second bypass line18, sample gas with aerosol particles can be passed directly to thesecond ionisation source 5. The third valve 33 enables to control thedirect flow of sample gas with aerosol particles to the secondionisation source 5.

The apparatus 1 comprises an electronic data acquisition system 41 forprocessing the signals provided by the mass analyser 3. This electronicdata acquisition system 41 comprises at least one analog-to-digitalconverter (ADC) producing digitized data from the signals obtained fromthe mass analyser 3 and a fast processing unit receiving the digitizeddata from said analog-to-digital converter, wherein the fast processingunit is programmed to continuously, in real time inspect the digitizeddata for events of interest measured by the mass analyser, and whereinthe electronic data acquisition system 41 is programmed to forward thedigitized data representing mass spectra relating to events of interestfor further analysis and to reject the digitized data representing massspectra not relating to events of interest. This mode is called “eventtrigger mode” and is described in WO 2016/004542 A1 of Tofwerk AG.

Furthermore, the apparatus 1 comprises a control unit 42 for triggeringthe mass analyser 3 whenever an individual aerosol particle reaches theextractive electronspray ionisation source 2, triggering the massanalyser 3 for analysing the ions originating from the individualaerosol particle. Thus, the apparatus 1 is a single particle aerosolmass spectrometer.

The invention is not limited to the embodiment described above. Forexample, the apparatus is not required to be a single particle aerosolmass spectrometer. For example, the apparatus may comprise an ionmobility spectrometer which is arranged between the extractiveelectronspray ionisation source and the mass analyser and fluidlycoupled to the extractive electronspray ionisation source and the massanalyser. In the embodiment shown in FIG. 1, the interface 4 may forexample comprise the ion mobility spectrometer or even be formed by theion mobility spectrometer.

For example the invention may be employed without one or more of thecalibration gas line, the second calibration gas line, the bypass line,the second bypass line, the test gas line, the clean gas line, thefilter line with the filter, the denuder line with the denuder, thevalve 31 and the second valve 32 and the third valve 33. Furthermore,the invention may be employed without the second ionisation source.

In summary, it is to be noted that an apparatus and a method foranalysing a chemical composition of aerosol particles are provided thatallow for a flexible use for analysing the chemical composition ofaerosol particles.

1. Apparatus for analysing a chemical composition of aerosol particles,the apparatus comprising: a) an extractive electrospray ionisationsource for extracting components, in particular organic compounds, fromthe aerosol particles and for ionising the components to ions, b) a massanalyser, in particular a time-of-flight mass analyser, for analysingthe ions, the mass analyser fluidly coupled to the extractiveelectrospray ionisation source, c) an inlet for passing a gaseoussuspension of the aerosol particles from an outside of the apparatusinto the apparatus for analysing the chemical composition of the aerosolparticles, the inlet being fluidly coupled to the extractiveelectrospray ionisation source, d) a denuder line which fluidly couplesthe inlet to the extractive electrospray ionisation source, the denuderline comprising a denuder for separating the aerosol particles from thegas phase of the gaseous suspension when passing from the inlet to theextractive electrospray ionisation source, and e) a filter line whichfluidly couples the outside of the apparatus with the extractiveelectrospray ionisation source, the filter line comprising a filter forfiltering the aerosol particles out of the gaseous suspension and thusseparating the gas phase from the aerosol particles when passing fromthe outside of the apparatus to the extractive electrospray ionisationsource, wherein the denuder line enables the aerosol particles to bypassthe filter, while the filter line enables the gas phase of the gaseoussuspension to bypass the denuder.
 2. Apparatus according to claim 1,wherein the mass analyser is configured to provide a mass spectrum ofthe ions when the ions are analysed.
 3. Apparatus according to claim 1,wherein the apparatus further comprises an electronic data acquisitionsystem for processing signals provided by the mass analyser, whereas theelectronic data acquisition system comprises a) at least oneanalog-to-digital converter producing digitized data from the signalsobtained from the mass analyser; b) a fast processing unit receiving thedigitized data from said analog-to-digital converter; wherein c) thefast processing unit is programmed to continuously, in real time inspectthe digitized data for events of interest measured by the mass analyser;and d) the electronic data acquisition system is programmed to forwardthe digitized data representing mass spectra relating to events ofinterest for further analysis and to reject the digitized datarepresenting mass spectra not relating to events of interest. 4.Apparatus according to claim 1, wherein the apparatus comprises an ionmobility spectrometer which is arranged between the extractiveelectrospray ionisation source and the mass analyser and fluidly coupledto the extractive electrospray ionisation source and the mass analyser.5.-7. (canceled)
 8. Apparatus according to claim 1, wherein theapparatus comprises a clean gas line for fluidly coupling a clean gassource via the denuder with the extractive electrospray ionisationsource.
 9. Apparatus according to claim 1, wherein the apparatuscomprises a test gas line for fluidly coupling a test gas source via thedenuder with the extractive electrospray ionisation source. 10.Apparatus according to claim 1, wherein the apparatus comprises an airline for fluidly connecting an air source with the extractiveelectrospray ionisation source.
 11. Apparatus according to claim 1,wherein the apparatus comprises a calibration gas line for fluidlyconnecting a calibration gas source with the extractive electrosprayionisation source.
 12. Apparatus according to claim 1, wherein theapparatus comprises a second ionisation source, in particular a sparkdischarge ionisation source, wherein the second ionisation source isparallel connected to the extractive electrospray ionisation source. 13.Apparatus according to claim 1, wherein the apparatus is an apparatusfor determining an aerosol particle size distribution and comprises anaerosol particle gate and a drifting region, wherein the drifting regionis fluidly coupled to the aerosol particle gate and the aerosol particlegate is configured to insert batches of aerosol particles or individualaerosol particles into the drifting region for passing the aerosolparticles through the drifting region, wherein the mass analyser isfluidly coupled to the drifting region and configured to measure thedrift time the aerosol particles require to pass through the driftingregion.
 14. Method for analysing a chemical composition of aerosolparticles, including the steps of: a) passing a gaseous suspension ofthe aerosol particles from an outside of an apparatus through an inletof the apparatus into the apparatus, separating the aerosol particlesfrom the gas phase of the gaseous suspension with a denuder of a denuderline of the apparatus, the denuder line fluidly coupling the inlet to anextractive electrospray ionisation source of the apparatus, andextracting components, in particular organic compounds, from the aerosolparticles with the extractive electrospray ionisation source andionising the components with the extractive electrospray ionisationsource to ions, or passing the gaseous suspension of aerosol particlesfrom the outside of the apparatus to the extractive electrosprayionisation source and filtering the aerosol particles out of the gaseoussuspension and thus separating the gas phase from the aerosol particleswith a filter of a filter line of the apparatus, the filter line fluidlycoupling the outside of the apparatus with the extractive electrosprayionisation source, and ionising the gas with the extractive electrosprayionisation source, b) transferring the ions to a mass analyser of theapparatus, in particular a time-of-flight mass analyser, the massanalyser being fluidly coupled to the extractive electrospray ionisationsource, and c) analysing the ions with the mass analyser, wherein thedenuder line enables the aerosol particles to bypass the filter, whilethe filter line enables the gas phase of the gaseous suspension tobypass the denuder.
 15. Method for detecting explosives in a samplecomprising aerosol particles, characterised in the steps of analysingthe sample with the method according to claim 14, and of examiningwhether the result obtained by the analysis with the method according toclaim 14 provides indications of the presence of an explosive.